1. Field of the Invention
This invention relates to light emitting diodes (LED) and in particular LEDs using lenses made of non-polymer materials.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangements, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light extraction, light focusing and the like. An LED package can also include electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. FIG. 1 shows a conventional LED package that generally comprises a single LED chip 12 mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup 13 can be filled with an encapsulant material 16 which can contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength can be absorbed by the phosphor, which can responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens or dome over the LED chip 12.
Conventional protective resins or lenses can be made of polymer materials such as silicones or epoxies, and the softening point for the polymer materials is relatively low. This allows softened polymer materials to be deposited directly on the LED without damage to LED metalized components, such as contacts, wire bonds, mirrors, reflective cups, leads, etc.
FIG. 2 shows another conventional LED package 20 that may be more suited for high power operations that can generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A reflector 24 can be included on the submount 23 that surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. Different reflectors can be used such as metal reflectors, omni-directional reflectors (ODRs), and distributed Bragg reflectors (DBRs). The reflector 24 can also provide mechanical protection to the LED chips 22. One or more wirebond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond. The encapsulant can be made of a relatively low melting point material to avoid damage to the metalized components of the package 20.
Many LED components for solid state lighting applications attempt to achieve high light output by operating single LED chips at as high as possible current and at a low voltage typical for individual LEDs. FIGS. 3a and 3b show one commercially available LED 30 available from Cree® Inc. under the EZ700™ LED product designation. The LED comprises a single LED junction 32 as well as a current spreading structure 34 on its top to spread current from the top contact 36. Current spreading layers can also be included. The particular voltage level for these types of single junction LED chips can be dependent upon the particular material system used for the LEDs and the voltage necessary based on junction voltage. For example, some Group-III nitride based LEDs can have junction voltage in the 2.5 to 3.5 volt range and increased luminous flux for these LEDs can be achieved by applying elevated current levels. LEDs such as these can also be provided with a lens or encapsulant as discussed above.
Operating an LED chip with high current can result at elevated chip temperatures, and heat from the LED chip can spread to surrounding parts of the LED package, including but not limited to the PCB, substrate or submount, as well as the lens or encapsulant (“lens”). Many lenses can be made of one or more polymer compounds such as epoxies and silicones. These can be fabricated in many different ways such as being formed directly over the LED, or formed separately and then mounted over the LED. For the desired operation, these lenses should maintain substantially the same transparency through their lifetime. However, high temperatures from the highly driven LEDs can spread to the lens, causing the lens heat-up. This can cause the polymer material of the lenses to degrade more quickly than would typically occur with operation under lower temperatures, and this degradation can result in certain undesirable effects for the LED package. In some cases the lens can become discolored or browned or even cracked, which can significantly reduce the transparency of the lens and can result in absorption of LED light passing through the lens. This in turn can reduce the overall emission efficiency of the package. This discoloration can also result in a shift in the color of light emitted by the package.
Lenses made of non-polymer materials such as glass, quartz and sapphire, can be more robust than lenses made of polymer materials, and can resist discoloration in response to elevated temperatures. It is not, however, practical to form these over conventional LEDs in the same way that polymer lenses are formed. Molding non-polymer lenses on an LED chip can present certain manufacturing challenges. In the case of glasses, the temperature for substantial softening can be approximately 400° C. or higher depending on the composition of the glass. Other crystalline transparent materials such as quartz have very high melting temperatures. These crystalline non-polymer lenses could be formed separately and then mounted over an LED with a polymer material such as a silicone or epoxy, but this presents the same potential problem of having a polymer material that can degrade when under elevated temperatures. The elevated temperatures can damage metalized components of the LED such as the contacts, wire bonds, reflectors, etc. The most severe damage is to the metal-to-semiconductor contact.